High-frequency generators are used to generate high-frequency power and to supply the high-frequency power to a load. Possible loads are, for example, plasma processes, such as plasma coating and plasma etching, or laser processes (laser excitation). Since the impedance of the load may change, and mismatching may thus lead to (partial) reflection of the power supplied by the high-frequency generator, it is often the case that not all of the power supplied by the high-frequency generator is absorbed in the load (e.g., the plasma). Determining the power absorbed by the load permits precise adjustments in the control of the high-frequency power supplied to the load.
A directional coupler may be used for measuring and/or determining the high-frequency power absorbed in a load. US Patent Publication 2005/0017821 A1, for example, discloses a directional coupler. The absorbed high-frequency power is derived from the difference between the power generated by the high-frequency generator and the reflected power. As a result, it is possible to control the high-frequency generator in such a way that the power absorbed in the load can be very precisely adjusted and can be kept constant.
A directional coupler including a main line via which high-frequency power is transmitted in the direction of the load and two secondary lines can be used to detect both the high-frequency power supplied in the direction of the load and the reflected power. In this case, the power supplied in the direction of the load can be measured by one secondary line and the reflected power can be measured by the other secondary line. As a result of the high-frequency power being supplied via the main line, electromagnetic fields occur that are coupled to the secondary lines such that a measurement signal can be detected at the secondary lines, which measurement signal is related to the power on the main line.
The quality of the measurement is described using the term “directivity”. The aim is to detect, on one secondary line, as far as possible, only portions of the power supplied in the direction of the load, and, by means of the other secondary line, as far as possible, only portions of the reflected power. In practice, however, this is not fully achieved. This means that the secondary line, by means of which only the power supplied in the direction of the load is intended to be detected, detects a small portion of the reflected power. The directivity refers to the relationship between the power detection of the desired signal and the power detection of the undesired signal. The directivity should be as great as possible.
Directional couplers are frequently implemented on circuit boards. However, directional couplers of this type are subject to manufacturing variations, the effects of which do not become apparent until after production when the function is being checked. If, in the case of directional couplers for multi-layered circuit boards, the coupling structure is on an inner layer, the geometry of the coupler structure cannot be adapted retrospectively, e.g. by means of laser trimming. Therefore, the tolerances have to be accepted, which leads to a lower measurement dynamic.